The referenced patents describe recording carriers and recording apparatus using the burn-off process of a metalized coating. Different grey-scale tones can be obtained by varying the voltage applied to the writing or recording electrodes. By changing the applied voltage, the size of the burned-out area is changed. If the image to be recorded is built as a composite of individual points of a pattern, the size of the points will vary.
The method as described is difficult to apply in actual practice, since the relationship between applied voltage and the degree of grey-scale toning is highly non-linear. Particularly if the voltage is low, a continuous relationship between voltage change and grey-scale reproduction cannot be unambiguously and reproducibly insured. With certain surface characteristics, and under low-voltage operating conditions, for example 18-20 V, the recording electrode operates in the form of a non-electric weld-type burner, which cuts out the writing trace from the recording carrier. In such operation, there is a tendency for metallic particles to adhere at the contact zone of the recording electrode. Non-metallic particles also may adhere. Consequently, formation of a recording track in which there is a predetermined relationship between applied voltage and size of the burn-out area cannot be obtained. If the voltage is increased, for example over 20 V, the formation of the writing trace changes. An arc forms between the recording electrode and the electrosensitive coating on the recording carrier. This arc, initially, has quite low energy content and is not capable of always completely vaporizing the metal which is to be removed. Consequently, the surfaces of the recording trace or, in the case of point recording, the various points of the pattern, do not reach values which have a predetermined correspondence with the amplitude of an applied recording signal; and, consequently, the grey-tone representation likewise will not unambiguously and reproducibly correspond to the applied signal. When the recording voltage exceeds a predetermined value, for example about 22 V, then the material to be removed is reliably completely vaporized. The process of burning-out of the recording track, up to complete black reproduction, will then be in accordance with the applied signals.
In various known processes, it is possible that oxide layers, which have been formed on the metallic coating of the recording carrier undesirably influence the quality of the reproduction of the image. Such oxide layers increase the transition resistance between the recording electrode and the metal coating and thus decrease the recording current or, in limiting cases, may even completely interrupt it.